1. Field
The invention relates to electronic controllers for sterilizers, and more particularly to memory backup in the event of power failures in such controllers.
2. State of the Art
Many electronic control devices depend on a random access memory (RAM) to provide information as to the progress through a control cycle. If the voltage to the RAM drops too low, as for example when a power failure occurs, the information accumulated in the RAM is lost. Then, when power is restarted, the unit must reset and begin its cycle over again. In a sterilizer device, such resetting means that the portion of the cycle already completed at the time of the power failure is wasted. It is therefore desirable that such controllers be able to retain data reflecting the progress through the immediate ongoing cycle in the RAM, so that the cycle may be continued from the point of interruption. On the other hand, if the power outage lasts too long there may be significant deviations from the temperature, pressure or other parameters prescribed for the portion of the cycle in which the outage occurs. In the latter case, to assure achieving sterility of the load it is better to restart the cycle than to continue from the interrupt point.
One approach to this problem is to have an alternate power source such as a battery available. However, batteries must be recharged or replaced at intervals. Record keeping is burdensome and employee time spent in such record keeping is expensive. Also, a low battery can result in failure to maintain the memory during an outage. By itself, a battery does not provide any means for determining whether the cycle data have been accurately retained.
A less expensive alternative is to include a capacitor circuit for temporary storage of charge from the main power source, as is described for an automatic cooking apparatus in U.S. Pat. No. 4,636,949 to Longabaugh. However, a capacitor will discharge fairly rapidly, and thus the voltage supplied therefrom may still drop below that required to assure retention of the cycle memory. Thus, it is still necessary to provide some means for determining whether the RAM data have been accurately retained. U.S. Pat. No. 4,750,040 to Hakamada discloses circuitry and programming for this purpose. However, a system such as that of Hakamada is too expensive and complicated to be suitable for a sterilizer system.
Accordingly, a need remains for means to retain cycle memory in sterilizers which is simple, inexpensive, and has the capacity to determine whether the cycle progress data have been accurately retained. A need moreover remains for such means which can discriminate between outages brief enough that the cycle can be continued from the point of interruption, and those in which restarting of the cycle from its beginning is needed to assure sterilization of the load.